Question

In my application I use ScheduledExecutorService, but only one thread is spawned to handle the scheduled tasks. Is this because ScheduledExecutorService does not spawn threads to handle the pending tasks?

Here is a code snippet that will output only “run() 1” instead of the expected “run() 1” followed by “run() 2” … “run() 10.”

public class App {

    public static void main(String[] args) {
        int N = 10;
        Runnable runner = new Runnable() {

            public void run() {
                foo();
            }
        };
        for (int i = 0; i < N; i++) {
            executor.schedule(runner, i, TimeUnit.MILLISECONDS);
        }
    }

    private static void foo() {
        System.out.println("run() " + (++n));
        synchronized (executor) {
            try {
                executor.wait();
            } catch (InterruptedException ex) {
                Logger.getLogger(App.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
        System.out.println("finished()");
    }
    private static Logger logger = Logger.getLogger(App.class.getName());
    private static int n = 0;
    private static ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
}

Answer 1

There is only one thread because you create the thread pool with Executors.newScheduledThreadPool(1), which means that the thread pool contains only 1 thread. If you want 10 threads, pass 10 as argument. Note that the documentation of ScheduledThreadPoolExecutor, which is what this method returns, explicitly says that the thread pool has a fixed size.